JP2000353936A - Low-pass filter - Google Patents

Abstract

PROBLEM TO BE SOLVED: To relax degradation of an amount of attenuation on a high-pass side and to relax restriction upon gain and noise characteristics of a differential amplifier by serially connecting a secondary filter circuit in front of an emitter follower type low-pass filter and using the filter circuit in common as a load of a differential amplifier. SOLUTION: A resistance 6 is connected to a front step of an emitter follower type low-pass filter, resistances 1, 3 and 5 are connected as a load of a differential amplifier, a capacitor 2 is connected between connection parts of the resistance 1 and the resistance 3 and a capacitor 4 between connection parts of the resistance 3 and the resistance 5. Also, a capacitor 4 is connected between the connection parts of the resistance 3 and the resistance 5. A transmission function becomes the one in which a 4-th low-pass filter circuit and a secondary high-pass filter are combined. Filter characteristics come to be attenuated by 12 dB/oct on the high-pass side. When a gain of a specific value is tried to gain, an emitter resistance can be made small and an NF of the differential amplifier can be improved.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a low-pass filter formed of, for example, an emitter follower and a differential amplifier, which is formed in, for example, a bipolar transistor semiconductor integrated circuit and the like, and more particularly, to improve deterioration of high-frequency attenuation characteristics. Then, the gain and noise characteristics of the differential amplifier are improved.

[0002]

2. Description of the Related Art A filter circuit using a resistor and an active element does not use an inductance, so that the circuit can be reduced in size and weight and integrated, and is widely used as a low-frequency pass filter of a radio device such as a mobile phone. .

As a filter of this type, a secondary low-pass filter circuit using an emitter follower has been conventionally known.

This circuit has an input terminal as shown in FIG.
19, an output terminal 18, an emitter follower circuit including a transistor 11 and a current source 15, a resistor 21 and a resistor 8 connected in series to an input terminal 19, and a resistor 10 connected in series to an output of the emitter follower circuit. A capacitor 7 is connected between a connection to the output terminal 11 and a connection between the resistors 21 and 8, and a capacitor 9 is connected between the input of the emitter follower circuit and ground.

Here, the resistance 10 is equal to the gm of the transistor 11.
1 / gm and the sum of the parasitic resistance at the emitter of the transistor.

FIG. 6 shows a case where this circuit is configured together with a differential amplifier. Since the impedance on the collector side of the transistor 13 of the differential amplifier becomes high impedance in a low frequency range, the circuit shown in FIG.
Can be shared as a load resistance of the differential amplifier, and the emitter follower circuit functions as both a buffer and a filter circuit of the differential amplifier. Therefore, the differential amplifier and the filter can be realized with a simple configuration.

[0007]

The signal input from the input terminal 19 of the circuit shown in FIG. 7 is Vin1 [dBm], the amplitude of the signal output from the output terminal 18 is Vo1 [dBm], and the cutoff frequency is Assuming fcl, the transfer function calculated by Vo1 / Vin1 is as follows. Vo1 / Vin1 = {(R8 × R10 × C7 × C9) S ² + (R10 +
C7) S + 1} / {C7 × C9 × (R21 × R8 + R8 × R10 + R
^{21 × R10) S 2 + (} R10 × C7 + R8 × C9 + R21 × C9)
S + 1}

The transfer function represented by the above equation is a combination of a second-order low-pass filter circuit and the reciprocal of the transfer function of the second-order low-pass filter circuit, ie, a second-order high-pass filter circuit. Can be seen. Assuming that the cutoff frequency of the second-order low-pass filter is fcl1 and the cutoff frequency of the second-order high-pass filter is fch1, they are expressed by the following equations. fcl1 = 1 / {2π (C7 × C9 × (R21 × R8 + R8 × R10 + R10 × R21) ^1/2 } (1) fch1 = 1 / {2π (R8 × R10 × C7 × C9)} (2)

At this time, the frequency f [H
z] when the signal Vin1 [dBm] is input
The amplitude Vo1 of the signal output from 8 is expressed by the following equation. Vo1 = Vin1−40 log (f / fc1) +40 log (f /
fch1) [dBm]

FIG. 8 shows the characteristics of this filter, and the filter characteristics deteriorate on the high frequency side.

As shown in FIG. 6, when it is configured with a differential amplifier, when integrated, capacitors C7 and C8
Needs to be set as small as possible in order to reduce the scale of the integrated circuit. Therefore, in order to obtain the same cutoff frequency, it is necessary to increase the resistances 8 and 21.

Further, since the resistor 21 is shared with the load of the differential amplifier, the value of the resistor 21 must be determined in consideration of the gain of the differential amplifier. For example, the cutoff frequency f
If you want to set cl1 to a low value, use capacitors C7 and C8
Since it is necessary to set as small as possible, the values of the resistors 8 and 21 are increased to determine the cutoff frequency.

However, if the resistance 21 is increased, the gain of the differential amplifier may be increased more than necessary, and it is necessary to adjust the gain by increasing the emitter resistance 14 of the differential amplifier. Become.

In general, the noise figure (NF) of a transistor circuit depends on the base resistance and the emitter resistance of the transistor. Therefore, if the emitter resistance 14 is increased, the noise characteristics of the differential amplifier, that is, NF is degraded.

If the resistance 8 is increased, the resistance 8 is inserted in series with the base of the transistor 11, so that the NF of the emitter follower circuit is deteriorated.

As described above, in the conventional circuit, the filter characteristic is degraded on the high frequency side, and the size of the capacitor that can be integrated is limited in terms of the scale. There is a problem that the gain and noise characteristics of the differential amplifier are limited due to the large dependence on the values of 8, 21.

The present invention solves such a conventional problem. In a low-pass filter composed of an emitter follower and a differential amplifier, the attenuation of the high-frequency side is reduced, and the cut-off frequency is reduced. The present invention provides a low-pass filter that can mitigate the limitation on the gain and noise characteristics of the differential amplifier due to the setting of the resistance without increasing the current or the circuit scale, and a semiconductor integrated circuit including the same. It aims to provide a mobile terminal and a communication system.

[0018]

Therefore, according to the present invention, in an emitter-follower type low-pass filter circuit combined with a differential amplifier, a secondary filter circuit is connected in series before the emitter-follower type low-pass filter. Further, the filter circuit is shared as a load of the differential amplifier.

This secondary filter acts to cancel the deterioration of the high-frequency attenuation of the low-pass filter circuit. Also,
Since the resistance of the secondary filter is divided and inserted as the load of the emitter follower and the differential amplifier, the load of the differential amplifier can be kept small, and the gain and noise characteristics of the differential amplifier can be regulated. Can be eased.

[0020]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS According to a first aspect of the present invention, in a secondary low-pass filter circuit having an emitter follower and a differential amplifier as components, a resistor and a resistor are provided before the low-pass filter circuit. A secondary filter composed of a capacitor is connected as a load of a differential amplifier, and this resistance is divided and inserted into a base input section of an emitter follower and a load of a differential amplifier. By the connection of the RC circuit, the deterioration of the attenuation in the high frequency band is canceled out, and only a part of the R becomes the load of the differential amplifier, so that the gain and the noise characteristics of the differential amplifier are defined. Restrictions are relaxed.

According to a second aspect of the present invention, there is provided a semiconductor integrated circuit incorporating the low-pass filter circuit according to the first aspect,
The circuit can be reduced in size and weight and integrated.

According to a third aspect of the present invention, there is provided a portable terminal incorporating the semiconductor integrated circuit according to the second aspect, and the terminal can be reduced in size and weight.

According to a fourth aspect of the present invention, there is provided a communication system including the portable terminal according to the third aspect and a base station connected to the portable terminal, and a communication system using a small and lightweight portable terminal can be constructed.

Hereinafter, embodiments of the present invention will be described with reference to the drawings.

As shown in FIG. 1, the low-pass filter circuit according to the embodiment of the present invention has a resistor 6 connected to a stage preceding a conventional emitter-follower type low-pass filter (FIG. 6), and a differential amplifier. Resistors 1, 3 and 5 are connected as a load, and a capacitor 2 is connected between a connection between the resistors 1 and 3, and a capacitor 4 is connected between a connection between the resistors 3 and 5.

As described above, the transistor 13 of the differential amplifier
If the differential amplifier performs a balanced operation when the impedance on the collector side is set to high impedance, the same signal voltage having a phase difference of 180 degrees is always applied to both ends of the capacitors 2 and 4. Therefore, the middle point of the capacitors 2 and 4 can be regarded as being grounded in an alternating current manner. Therefore, FIG. 1 can be represented by the circuit of FIG. Here, in the circuit of FIG. 2, the capacitor C22 is C2 / C2, the capacitor C23 is C4 / C2, and R20 is R5.
+ R6.

The amplitude of the signal input from the input terminal 19 of the circuit shown in FIG. 2 is Vin [dBm], the amplitude of the signal output from the output terminal 18 is Vo [dBm], and the transfer function Vo / dBm
Find Vin. Here, for example, in order to simplify the calculation, in the circuit of FIG. 2, C = C22 = C23 = C7 = C9,
When the transfer function is obtained under the condition of R = R1 = R3 = R20 = R8 = R10, the following equation is obtained. Vo / Vin = {(C ² × R ² ) S ² + (C × R) S + 1} /
{(2 × C ⁴ × R ⁴ ) S ⁴ + (12 × C ³ × R ³ ) S ³ + (15 × C
² × R ² ) S ² + (20 × C × R) S + 1}

This transfer function Vo / Vin is a combination of a fourth-order low-pass filter circuit and a second-order high-pass filter. Assuming that the cutoff frequency of the fourth-order low-pass filter circuit is fcl and the cutoff frequency of the second-order high-pass filter is fch, fcl and fch are expressed as follows. fcl = 1 / {2π (2 × C ⁴ × R ⁴ ) ^1/4 } (3) fch = 1 / {2π (R ² × C ² ) ^1/2 } (4)

At this time, the frequency f [H
z] when the signal Vin [dBm] is input
The amplitude Vo of the output signal is represented by the following equation. Vo1 = Vin1-80log (f / fcl) + 40log (f / f
ch) [dBm]

FIG. 3 shows the characteristics of this filter. The filter characteristics are attenuated at 12 dB / oct on the high frequency side.

Here, for example, R = R1 = R3 = R5 = R6 = R8 = R10, C7, C9≫C2
If C2 and C23 are used, the impedance of C23 and C24 becomes large in the low frequency range. In FIG. 1, the connection between the connection between the resistance 1 and the resistance 3 and the connection between the connection between the resistance 3 and the resistance 5 can be regarded as open. Therefore, the circuit of FIG. 1 can be represented as shown in FIG.
Therefore, the circuit of FIG. 2 can be represented as shown in FIG. The circuits shown in FIGS. 4 and 5 are equivalent to FIGS. 6 and 7 if R21 = R1 + R3 + R5 + R6.

For example, assuming that R21 = Ra in FIG. 6, if R1 = R3 = R5 = R6 = Ra / 4 in FIG. 4, the load resistance of the differential amplifier becomes (3 × Ra) / 4. When trying to obtain the same gain as the circuit of FIG. 6, the emitter resistance R14 can be reduced, and the NF of the differential amplifier can be improved.

At this time, a resistor of Ra / 4 is inserted in series with the base of the emitter follower, and the NF of the emitter follower is deteriorated. However, the resistance value is as small as Ra / 4, and the NF of the entire circuit is reduced. Generally, if the gain of the first-stage differential amplifier is high, the NF of the differential amplifier becomes dominant, so that the deterioration of the NF of the emitter follower can be ignored with respect to the NF of the entire circuit.

As described above, in the emitter follower type low-pass filter circuit combined with the differential amplifier of the present invention, the RC circuit is inserted as the load of the differential amplifier, and the resistor R is connected to the load of the differential amplifier and the emitter follower. By dividing the input into the base input, it is possible to alleviate the deterioration of the attenuation in the high frequency range, and at the same time, to reduce the restrictions in defining the gain and noise characteristics of the differential amplifier without increasing the current or the circuit scale. Can be.

Further, by incorporating this semiconductor integrated circuit into a wireless device such as a mobile phone, the size and weight of the wireless device can be reduced. Further, the convenience of the communication system using the wireless device can be improved.

[0036]

As is apparent from the above description, the emitter-follower type low-pass filter circuit combined with the differential amplifier of the present invention can alleviate the deterioration of the high-frequency attenuation and at the same time reduce the gain of the differential amplifier. And the limitation of the constant in defining the noise characteristics can be relaxed without increasing the current or the circuit scale.

The filter circuit can be integrated with other circuits to form an integrated circuit. By incorporating this integrated circuit into a wireless device, the size and weight of the wireless device can be reduced.

[Brief description of the drawings]

FIG. 1 is a circuit diagram of a low-pass filter according to an embodiment of the present invention;

2 is an equivalent circuit diagram of the low-pass filter of the present invention in FIG. 1,

FIG. 3 is a frequency characteristic diagram of the low-pass filter of the present invention

FIG. 4 is an equivalent circuit diagram of FIG. 1 in a low frequency region when capacitors C7 and C9≫C2 and C4 are set;

FIG. 5 is an equivalent circuit diagram of FIG. 4;

FIG. 6 is a circuit diagram of an emitter-follower type low-pass filter combining a conventional differential amplifier,

FIG. 7 is a circuit diagram of a conventional emitter follower type low-pass filter,

FIG. 8 is a frequency characteristic of a conventional emitter follower type low-pass filter.

[Explanation of symbols]

 1, 3, 5, 6, 8, 10, 14, 20, 21 Resistance 2, 4, 7, 9, 22, 23 Capacitors 11, 12, 13 NPN transistor 15 Constant current source 18 Output terminal 19 Input terminal

Claims (4)

[Claims] 1. A second-order low-pass filter circuit having an emitter follower and a differential amplifier as constituent elements, wherein a second-order filter including a resistor and a capacitor is provided at a stage preceding the low-pass filter circuit. A low-pass filter circuit connected as a load of an amplifier, wherein the resistor is divided and inserted into a base input of the emitter follower and a load of the differential amplifier. 2. A semiconductor integrated circuit incorporating the low-pass filter circuit according to claim 1. 3. A mobile terminal incorporating the semiconductor integrated circuit according to claim 2. 4. A communication system comprising the mobile terminal according to claim 3 and a base station connected to the mobile terminal.